18K Ni-FREE AGE HARDENABLE WHITE GOLD ALLOY

ABSTRACT

The present invention is directed to a reduced-cost 18K white gold alloy that is Ni-free, and otherwise contains a low w % of precious metals, including about 7 w % palladium, exhibits a color which is considered white, and shows age hardening characteristics commensurate with nickel containing 18 Karat gold alloys.

BACKGROUND

In some metal jewelry applications, such as in preparing a ring, a lighter than usual color of gold is desirable. One technique to lighten the color is often termed “bleaching”. There are two elements, nickel (Ni) and palladium (Pd), which are commonly used to bleach the color of gold when preparing white gold alloys for jewelry applications. Ni and Pd are preferably used because both metals exhibit bright grey color, and both have the same face-centered cubic crystalline structure as other major alloying elements such as gold, silver and copper.

In the past decade or so, concern has grown with regard to the inclusion of Ni in jewelry. The increased concern about nickel as a cause of allergy has led to the development of Ni-free Pd-containing white golds alloys.

By definition, 18 karat gold alloys must contain 75% gold by weight (w %) minimum to be considered 18 karat, leaving only 25 w % for all other elements, including other major elements such as silver (Ag), copper (Cu) and, in the case of white gold, Pd. In Ni-free, Pd-containing alloys, Pd tends to be in the approximate range of 6 w % 15 w %. The increasing price of precious metals including Pd makes it desirable for Ni-free white gold alloys to contain as little Pd as possible while still retaining the color and durability benefits of Pd-containing white gold alloys. However, a too-limited percentage of Pd, especially when combined with the elimination of Ni in the alloy, leads to the deterioration of the white color overall.

The whiteness of the color of gold alloys can be graded using the yellowness index (YI) as described in “White Gold Alloys: Colour Measurement and Grading” by Steven Henderson and Dippal Manchanda in Gold Bulletin, 2005, vol. 38, issue 2, pp. 55-67. According to this reference there are three grades of alloys which can be considered white in color:

TABLE 1 Grade: Whiteness YI Rhodium Plating Grade 1: Good White YI < 19.0 Not necessary Grade 2: Reasonable White 19.0 < YI < 24.5 Optional Grade 3: Off-White 24.5 < YI < 32.0 Recommended

EXAMPLES

One commercially available 18K alloy that contains 15% palladium shows a good white color with YI=18.5.

The color of the alloy that contains 12% palladium (alloy 13 in Table I in U.S. Pat. No. 6,787,102 to Vincent) is reasonable white with YI=19.5.

The color of the alloy that contains 7% palladium (alloy 7 in Vincent's patent) shows off-white color with YI=31.1.

For comparison, a commercially available 18 k yellow alloy containing no palladium has YI=48.5

Alloys with VI>32.0 cannot be called white.

Vincent teaches a nickel-free grey alloy that contains 75 w %-76 w % gold, up to 13 w % Pd, up to 10 w % Cu, some other additions such as gallium and indium, and no silver. Even though an alloy with 13 w % Pd shows a reasonable white color, it is expensive due to the high Pd content. In the description of Vincent, Table 2 lists some alloys that contain 7 w % Pd. Three of these alloys are listed in Table 2 below.

TABLE 2 VH Vh Alloy Au Pd Ag Cu Zn Ir In Re Li Other L* a* b* YI ann aged U.S. Pat. No. 75 7 7 7 2.5 0.01 1.2 0.002 balance 85.4 1.79 15.04 31.4 150 5,787,102, Table II alloy 5 U.S. Pat. No. 75 7 11 7 0.01 0.002 balance 83.7 3.06 14.02 31.1 222 6,787,102, Table II, alloy 7 U.S. Pat. No. 75 7 0.9 10 7 0.01 0.002 balance 83.2 2.79 14.26 31.5 155 6,787,102, Table II, alloy 8 Present 75.2 7 4.88 10 2.5 0.02 0.4 Co 82 2.1 12.9 28.7 155 200 invention preferred composition Present 75 6 3 10.6 5.1 0.3 Co 80.0 2.2 12.9 29.3 180 240 invention, alternate composition

Alloy 5 contains 75 w % gold, 7 w % Pd, 7 w % copper, 2.5 w % zinc, and small additions of iridium (Ir), indium (In) and rhenium (Re). The CIELab color coordinates L*, a* and b* are 85,4, 1.79 and 15.04 respectively. The calculated yellowness index YI is 31.4. This YI value is very close to the border line between white and non-white colors. The annealed hardness is listed as 150 Vickers.

Alloy 7 contains 75 w % gold, 7 w % palladium, 11 w % copper, 7 w % zinc, and small additions of iridium (Ir) and rhenium (Re). The CIELab color coordinates L*, a* and b* are 83.7, 3.06 and 14.02 respectively. The calculated yellowness index YI is 31.1. This YI value is also very close to the border line between white and non-white colors. The annealed hardness is listed as 222 Vickers.

Alloy 8 contains 75 w % gold, 7 w % palladium, 0.9 w % silver (Ag), 10 w % copper, 7 w % zinc, and small additions of iridium (Ir), indium (In) and rhenium (Re). The CIELab color coordinates L*, a* and b* are 83.2, 2.79 and 14.26 respectively. The calculated yellowness index YI is 31.5. Again, this YI value is very close to the border line between white and non-white colors. The annealed hardness is listed as 155 Vickers.

Further, the whiteness in some of the alloys of Vincent, particularly the alloys shown in Table 1, is close to the borderline in what can be classified as white. This can be a concern because color properties of an alloy may vary from batch to batch due to a variety of factors, and when a color is close to a border, at times the color variability may cause a particular formulation to fall outside the white range. As a result, it is desirable to form an alloy which provides better assurance of being considered white and meets other characteristics of hardenability and cost while remaining nickel free.

Age hardening is beneficial because such alloys can be easily worked and formed in the annealed condition into the jewelry article, and then the finished jewelry article can be age hardened. Age hardened jewelry shows higher durability and polish. Age hardening also allows the manufacturing the light weight, thin wall and reduced cross section jewelry articles without compromising the strength and durability, This provides additional savings on the material cost. Even though Vincent does not teach hardenability or age hardening, the alloys 7 and 8 most likely cannot be age hardened by a heat treatment as they contain either no silver or very little silver.

DESCRIPTION OF THE INVENTION

It is an objective of the present invention to provide a reduced-cost 18K white gold alloy that is Ni-free, and otherwise contains a low w % of precious metals, including about 6-7w % palladium, exhibits a color which is considered white, and shows age hardening characteristics commensurate with nickel containing 18 Karat gold alloys.

It is a further objective of the present invention to provide an 18K gold alloy for jewelry whereby the color of the alloy falls within the whiteness portion of the YI scale, contains no Ni, has an overall reduction in precious metal content so as to save costs of materials, includes certain preferred hardness attributes, and is durability.

In general, the present invention is used to result in a composition with Grade 3 of whiteness by using about 6-7% of Pd and including other precious metals, while maintaining annealed hardness factor of about 160 Vickers, and aged hardness of 190-200 Vickers.

The preferred composition of the alloy of the present invention is listed in Table 2, This alloy contains 75.2 w % gold, 7 w % palladium, 4.88 w % silver, 10 w % copper and 2.5 w % zinc; 0.02 w % lithium may be added to the alloy as a deoxidizer, and about 0.3% cobalt may be added as a grain refiner. The alloy shows the following CIELab color coordinates; L*=82, a*=2.1 and b*=12.9. In general, whiteness on the CIELab coordinates requires at least Grade 3 white, and preferably in the middle or better of the range. In this case, the alloy can be considered Grade 3 white.

In addition, the calculated yellowness index YI=28.7. This YI value indicates a significant improvement of white color as compared with the alloys of Vincent.

The color of the alternate composition is closer to borderline white, the annealed hardness is a little higher than optimal, and material is not as workable as that with 7% Pd.

In general, the preferred range of materials in the 18 karat white gold alloy is 74.5%-75.5% by weight gold, 6%-8% by weight palladium, 3.5%-5.5% by weigh silver, 2.0%-3.0% by weight zinc, 0.0-0.03% by weight lithium, 0.0-0.5% cobalt and the balance copper. Preferably, the alloy's YI is below 29.

In the present invention, lithium is used to attract oxygen that can be potentially released by silver and palladium during melting. Cobalt is used to reduce the grain size of the alloy during annealing and re-crystallization.

Pd is used, nominally at 7% by weight, to provide both hardness and color to the resultant alloy. The more Pd, the whiter in color the alloy will be. However, Pd must be limited because too much Pd increases cost and can adversely affect the ability of forming the alloy. In the preferred embodiment, 7% Pd by weight is preferred.

The color is formed due to presence of 7% palladium in combination with about 5% silver. More Pd would improve the whiteness of the color, but results in increased material cost. To obtain Grade 2 white about 12% palladium is needed to be present in the alloy.

Silver is also useful for improving whiteness in an 18 karat gold alloy, particularly when the cost of silver makes it beneficial. However, silver has drawbacks, such as it is less hard than Pd. Consequently, different mixes of Pd and Ag can be used for whiteness.

Zinc is included for several fundamental purposes—because of its comparatively low cost, it reduces the overall cost of the alloy, to aid in hardness of the alloy, and for purposes of color management.

The annealed hardness of the alloy of the preferred composition is about 155 Vickers. The alloy should be age hardened by heat treatment at about 700° F. for about 1 hour up to at least 200 Vickers. This is a procedure for age hardening.

Once the alloy is formed, it is finished with Rhodium plating, as Rhodium plating is required for Grade 3 white gold alloys.

Data regarding the preferred embodiment is included below. The product, entitled 18K LG460 White, is nickel free. It includes 75.2% Gold, 4.9% Silver, and 7% Palladium. The density is 162.5 dwt/in³ (15.4 g/cm³). The color is white (Rh plating is recommended). The Red-Green a* is 2.1. The Yellow b* is 12.9. The annealed hardness is 155 Vickers and the aged hardness is 200 Vickers. The annealing temperature is 1250° F. (675° C.) and the aging temperature is 700° F. (370° C.). The melting range (Solidus-Liquidus) is 1645° F. (895° C.)-1725° F. (940° C.). The annealing at 1250° F. (675° C.) must be done in the protective atmosphere for 0.5 hr to 1 hr depending on the material mass. Age hardening should be done at 700° F. (370° C.). is for 1 to 2 hours in the protective atmosphere. When age hardening is done using a belt furnace, the item should be kept in the hot zone for about 2 hrs. This can be achieved by stopping the belt. After hardening, the material may be quenched in water or allowed to cool in the water-cooled chamber of the belt furnace. The age hardening is normally performed prior to the final finishing operation. 

1. A gold-based nickel-free alloy composition comprising: about 75% gold, about 6-7% palladium, about 10% copper, and 2-5.5% zinc; wherein said composition exhibits a color which is considered white, and shows age hardening characteristics commensurate with nickel containing 18 Karat gold alloys.
 2. The composition of claim 1 further comprising about 3% silver.
 3. The composition of claim 1 further comprising about 5% silver.
 4. The composition of claim 1, wherein the age hardness as cast exceeds 155 vickers.
 5. The composition of claim 1, wherein the age hardness as cast exceeds 175 vickers.
 6. The composition of claim 1, wherein the age hardness increases by at least 50 vickers when heated.
 7. The composition of claim 1 wherein color components L*, *, and b* are in the ranges of 80-84, 2-2.5, and 12.75-13, respectively.
 8. The composition of claim 1, wherein the YI value is in the range of 28.7-29.3.
 9. The composition of claim 1, wherein said composition is used for jewelry.
 10. The composition of claim 9, wherein said composition is absent any rhodium plating. 